A simple foundation
There’s no need to pour concrete. Get off the ground with a simple treated-wood foundation. Build it just like a deck platform.
Match the siding and roof
Walls and roof finish the addition and make it look like it’s always been part of the house.
Is a packed garage keeping you from buying your dream motorcycle? Or maybe you just need a little more space for lawn equipment, bikes or your woodworking tools. A bump-out addition may be the solution to your overcrowded garage. And in this article we’ll show you how to build one. If you have basic carpentry skills and experience building a deck, shed or other structure, then you can build this garage addition. In this article, we’ll focus on the tough parts of the project, like installing the beam and building the roof. Study Figures A – C for construction details. A Materials List is available in Additional Information below.
Figure A: Bump-Out Details
Start with a traditional pressure-treated deck, then add the walls and roof.
Tools, time and cost
In addition to a basic set of carpentry tools, you’ll need a hammer drill to bore holes for the concrete anchors, and ladders or rented scaffolding to finish the roofing and other exterior details. While not essential, a reciprocating saw will simplify the wall tear-out.
We spent roughly $2,000 for materials for this 5 x 12-ft. garage addition. Your costs will vary depending on the type of siding, roofing and windows you choose.
With a helper, plan to spend about half a day installing the beam and a long weekend framing the addition. Then plan to spend another weekend finishing the exterior. The time it takes to complete the project will depend on whether you finish the inside as we did. Keep in mind that you can hire a siding, roofing or drywall contractor to complete parts of the project you’re not comfortable with.
This addition is large enough to accommodate a garden tractor, a large motorcycle, or a workbench and a table saw. If you’d prefer a larger size, contact an architect or structural engineer to specify the size of the header, floor joists and rafters.
Garage additions usually require a building permit. Contact your local building department to see what’s needed. Make sure to call 811 a few days before you dig the footing holes to have underground utilities located and marked.
Support the roof with a new header
Photo 1: Build the header
Frame the sides of the opening and install the new header. Then support the header with double trimmers on each end.
Figure B: Header Details
Set the new header on the cut-off studs, then force it up tight to the plate with shims before installing the new trimmer studs.
Before removing a section of the garage wall, you’ll have to add a header to support the weight of the roof above it. The gable end of this truss-framed garage roof doesn’t support much weight, so we were able to add the double 2×10 header and remove the wall section without adding temporary supports. Gable end walls that support a second floor or ridge beam will require a larger header and temporary support. If you have a second floor above the garage or aren’t sure how the roof is framed, contact an architect or engineer to calculate the header size.
Start by locating the center of your addition and marking the size of the opening on the bottom plate of the wall. We centered the bump-out on the garage wall, but this isn’t necessary. Remember to make the opening 7 in. narrower than the width of the addition. Next make marks 3 in. beyond the opening marks on each side to locate the inside edge of the king studs. Cut king studs and nail them to the top and bottom plates. For a 12-ft.-wide addition, the distance between the king studs should be 11 ft. 11 in. Cut the 2×10 headers to this length and nail them together with 10d nails. Nail a 2×4 to the bottom of the 2x10s to complete the header (Figure B).
Cut out the top section of the wall studs with a reciprocating saw to make a path for the new header. Saw the studs 11-1/2 in. down from the top plate and knock out the short stud sections with a hammer. Then cut off the nails with a nail nipper or reciprocating saw. You may also have to cut off a few sheathing and siding nails so they don’t interfere with the header. After nailing in the king studs (Photo 1), set the header in place on the cutoff studs. Wedge the header tight to the top plate by driving shims between the cutoff studs and the header at each end. Cut a pair of trimmer studs for each end of the header and nail them in place with 16d nails (Photo 1).
Remove the wall
Photo 2: Demolition
Knock out the wall to create the new opening. First cut the sheathing and bottom plate along the framed opening. Then cut the wall into sections and push them out.
If you have vinyl, aluminum or steel siding, it’s probably easier to remove it from the entire garage wall and reinstall it after the addition is built. We cut the wood siding 2-1/2 in. beyond the width of the addition to allow for 1/2-in. wall sheathing and 2-in. inside corner boards. Pry off the siding and remove the wall (Photo 2).
In many garages, the walls rest on a block or concrete curb that has to be removed to create a continuous floor. We used an angle grinder fitted with a diamond blade to score the concrete block flush to the cutoff bottom plate on both sides of the opening. Then we broke out the concrete blocks between the trimmers with a sledgehammer. If you have a solid concrete curb, removing it will be a tougher job. Consider renting a concrete saw to score the entire length of the opening before breaking it out. Photo 6 shows how to patch the concrete after the plywood floor of the addition is in place.
Build the platform
Photo 3: Ledger board
Attach the ledger board to the garage foundation with concrete sleeve anchors. Temporarily support the ledger with wood stakes while you drill holes for the anchors.
Close up of anchor
Use a sleeve anchor or other heavy-duty anchor to hold the ledger.
Photo 4: Establish the post height
Mark the posts level with the bottom of the ledger and cut them off. Put the cutoff posts back in the holes, but don’t fill around them until you’ve built and squared the floor frame.
Photo 5: Build the floor
Start the floor frame by toenailing the joists to the ledger and nailing through the first rim joist into the floor joists. Then install the second rim joist and nail on the joist hangers. Square the frame and nail the two layers of plywood to the joists.
Photo 6: Fill gaps
Fill the gap between the garage floor and the plywood bump-out floor with concrete. Drag a scrap of 2×4 over the concrete to level it. Then trowel it smooth when it begins to harden.
Start building the platform by bolting the ledger to the garage. Position the top edge of the ledger 1-1/2 in. below the garage floor in the center of the opening and level it, using temporary stakes for support. Attach it with 1/2-in. concrete sleeve anchors located 12 in. apart. Add two extra anchors at each end.
Next locate and mark the center of the footings (Figure A). Dig the footing holes and pour 8-in.-thick concrete pads into the bottom of each. After the concrete hardens, set treated 4×4 posts into the footing holes. At this point, you’ll know whether you need to remove soil in the area under the bump-out to make room for the joists. Make sure there are at least a few inches of clearance between the bottom of the joists and the ground.
The next step is to cut the posts to the correct length and build the platform. Use a level resting on a straight board to mark each post level with the bottom of the ledger board (Photo 4). Cut the posts at the marks and set them back into the holes. Complete the floor frame by cutting and assembling the floor joists and attaching the frame to the posts with metal post caps (Figure A). Finally, pack dirt around the posts and nail one layer of treated plywood and a second layer of BC plywood to the floor framing. If the surrounding soil is above the bottom of the joists, build a three-sided 2×6 dam and pack the soil against it. Don’t attach the 2x6s to the joists. The goal is to prevent dirt from getting under the joists while allowing the 2x6s to move with the soil.
Build and stand the walls
Photo 7: Add the walls
Build the walls on a flat surface like the garage floor. Stand them up on the bump-out floor and nail them to the floor and to the garage wall. Plumb and brace the front wall before installing the sheathing.
Build the walls on a flat surface like the garage floor. Start by cutting the top and bottom plates for the long wall and marking the stud locations on them. Use Figure A as a guide. Lay out the window opening to match the rough-opening dimensions provided by your window supplier. Assemble the wall by nailing the studs to the plates with 16d nails. Stand the wall on the platform, straighten the bottom plate and nail it to the rim joist. Plumb and brace the outside corners. Now measure from the new front wall to the existing garage wall at the top and bottom on each side and cut the plates for the short walls accordingly. If the existing garage wall isn’t plumb, this procedure will ensure that the short walls will fit correctly.
Build the two short walls and set them in place on the platform. First position the walls and nail through the bottom plate into the floor with 16d nails. With a level, plumb the stud that’s against the garage wall before nailing it to the wall. Align the corner studs on adjacent walls and nail them together. Finally, use a level to make sure the corner stud on the long wall is plumb and nail a diagonal brace to the inside of the wall (Photo 7). Complete the wall construction by adding the tie plates, making sure to cut them so they overlap the top plate at the corners.
Figure C: Rafter Details
It’s easier to visualize and mark the roof rafter cuts if you mark them in place on the wall.
Cut the rafters and build the roof
Photo 8: Lay out the roof
Make a rafter pattern by tacking a 2×6 to the garage wall, parallel to the garage roof, and drawing lines on it to indicate the center of the bump-out, the overhang distance and the bird’s-mouth cut. Use the pattern to mark and cut the rafters.
Photo 9: Cut the siding
Tack a pair of rafters to the garage wall as a guide for cutting the siding. Place a 2×4 spacer between the top ends of the rafters to simulate the ridge. Remove the rafters and pry off the siding.
Photo 10: Frame the roof
Temporarily support the ridge while you nail the rafters to it. Nail through the ridge into the rafters and toenail the rafters to the tie plate. Then reinforce the connection between the rafters and the tie plate with metal hurricane ties.
Photo 11: Build the gable end
Complete the roof frame and fill in the gable end rafters. Then nail plywood over the roof frame.
For the best appearance, match the slopes of the addition and garage roofs. In Photo 8, we show a simple method of marking a 2×6 to use as a guide for making a pattern rafter. Figure C shows how to modify the marks to create a pattern rafter. Use the pattern to mark the remaining rafters. Photo 9 shows how to cut the siding using the rafters as a saw guide. We added a 3/8-in. shim under the rafters to allow a 1-3/4-in. gap between the roof framing and the siding cut (Photo 9). This provided enough room for 1/2-in. sheathing, two layers of dimensional shingles and a 3/4-in. space for the step flashing to slide into. Adjust the cut in your siding to correspond to the thickness of your roofing and sheathing material. Set the saw just deep enough to cut through the siding. When you’re done cutting both sides, remove the rafters and pry off the siding in the area of the new roof.
Frame the roof using Figure A and Photos 10 and 11 as a guide. Nail through the ridge into the rafters and toenail the bird’s-mouth to the tie plate. Reinforce the connection between the rafters and the tie plate with metal hurricane ties. Then complete the roof frame by adding the 2×6 subfascia and building the side and end overhangs. Match the overhangs to the overhangs on the garage. When you’re done with the roof frame, cut 1/2-in. sheathing to fit and nail it to the rafters.
Finish the addition with trim, roofing and siding
Photo 12: Place windows
Install the windows according to the manufacturer’s instructions. Layer self-adhesive flashing over the nailing fins as shown, making sure each layer overlaps the one below and that the final assembly will shed water over the top of the building paper below the window.
Photo 13: Flash and shingle
Shingle the roof. Install building paper and shingles according to local codes and the instructions on the shingle package. Slide galvanized metal step flashing under the siding and position it over the end shingle in each course.
At this point in the project, your garage addition will probably vary considerably from what we show here. In general, you’ll start by finishing the trim on the overhangs, including the soffit and fascia, with wood or metal to match your garage. Then install the roof shingles according to the manufacturer’s instructions. The key to a leak-proof roof is proper step flashing (Photo 13). We slid the flashing under the siding. But if you’ve removed the siding entirely, then simply install the step flashing with the shingles. Then install the siding over the flashing.
Install the window before the siding, being careful to flash around it with building paper or special self-adhesive window- flashing tape according to the manufacturer’s instructions (Photo 12). Finally, install siding to match your garage.
On the inside we added a few outlets and recessed ceiling lights. Then we insulated the walls and ceiling before hanging and taping the drywall.
Required Tools for this Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
- Angle grinder
- Circular saw
- Cordless drill
- Dust mask
- Extension ladder
- Framing square
- Miter saw
- Posthole digger
- Reciprocating saw
- Safety glasses
- Socket/ratchet set
- Utility knife